1. Field of the Invention
The present invention relates to power supply systems including a DC power supply string in which a direct current (DC) power supply for outputting a DC power from a power generator such as solar battery, wind power generator, and fuel battery and a storage battery are connected in parallel; in particular, to a power supply system for smoothing the power variation of the DC power supply and time shifting the output power of the DC power supply.
The present invention also relates to a power supply system for converting the output power of the DC power supply which generated power amount greatly varies according to environment without requiring a complex control. The present invention also relates to a power supply system capable of supplying maximum power by equalizing each battery voltage so that one of a plurality of storage batteries is not overly used, and obtaining a total power of each storage battery.
2. Description of the Related Art
The output power generated by the power generator such as solar battery, wind power generator, and fuel battery can be supplied to an electrical load or reverse flowed to a commercial power source. In the prior art, however, the output of the solar battery, the wind power generator, or the fuel battery is stored in the storage battery, and collectively converted with the power of the storage battery in a power conditioner, and thus the conversion power of the power conditioner needs to be limited to avoid power flow-out from the storage battery to the commercial power source. The limitation of the conversion power of the power conditioner results in limiting the generating power of the solar battery, the wind power generator, or the fuel battery, and thus maximum output of the solar battery, the wind power generator, or the fuel battery cannot be retrieved as a result, and the generated power output cannot be sufficiently reverse flowed.
In order to solve such problem, a power storage photovoltaic system of Japanese Laid-Open Patent Publication No. 2002-171674 is equipped with a photovoltaic power conditioner having a first power conversion means which performs cooperative operation with a power system connected between a solar battery and a power system, and having a second power conversion means connected between a power storage means and the first power conversion means. A load is connected between the power conditioner and the power system, and a control means for controlling the second power conversion means so that a receiving power does not become less than a predetermined power in time of power output from the power storage means by a receiving power detection means connected between the load and the power system is arranged.
According to Japanese Laid-Open Patent Publication No. 2002-171674, the output of the storage battery is adjusted with the second power conversion means, and the output from the storage battery can be stopped when reverse flowing power by photovoltaic generation to the power system while efficiently discharging the midnight power stored in advance.
A method of performing a maximum power point tracking control operation to retrieve a maximum power from the solar battery, and connecting the solar battery and the storage battery in parallel and performing a storage battery combined operation without a troublesome switching operation is disclosed in Japanese Laid-Open Patent Publication No. 2002-34175. According to this system, the power of the storage battery can be output when the generated power amount of the solar battery is significantly low.
A technique of utilizing the power generating ability of the solar battery to a maximum extent, an insolation energy effectively and extending the lifespan of the battery is disclosed in Japanese Laid-Open Patent Publication No. 6-266458.
The systems of Japanese Laid-Open Patent Publication No. 2002-34175 and Japanese Laid-Open Patent Publication No. 6-266458 are provided to operate the photovoltaic generation at a maximum power and retrieve the generated power to a maximum extent, and are systems mainly focused on performing maximum power point tracking irrespective of the power variation by the insolation variation.
Japanese Laid-Open Patent Publication No. 2001-224142 relates to a photovoltaic device which operates a storage battery combined operation including a plurality of strings consisting of a solar battery and a secondary battery and an operation of only the solar battery without a troublesome switching operation, preferentially charges the storage battery during the day, and maintains the storage battery in a fully charged state as much as possible to prepare for autonomous operation. The device in Japanese Laid-Open Patent Publication No. 2001-224142 will be described using FIG. 31.
As shown in FIG. 31, each string 120a, 120b including a solar battery 121a, 121b and a secondary battery 122a, 122b respectively includes an inverter 125a, 125b. The generated power outputs of the solar batteries 121a and 121b are supplied to the inverters 125a and 125b when solar light is sufficiently irradiating during the daytime. If solar irradiation lacks, the powers are supplied from the secondary batteries 122a and 122b to the inverters 125a and 125b. Each inverter 125a, 125b is operated in accordance with the power demand of a load 126 to supply the necessary power. Since the inverter 125a, 125b is arranged for every string, the input voltage of the inverter 125a, 125b, that is, the voltage of the solar battery 121a, 121b and the secondary battery 122a, 122b is independently set for every string.
In charging to the secondary batteries 122a, 122b, a switch for bypassing to backflow prevention diode 124a, 124b on the secondary battery 122a, 122b side is arranged in advance, and a bypass switch of the storage battery to be charged when solar irradiation is sufficient is turned ON to charge from the solar batteries 121a, 121b. 
FIG. 32 shows another example of the conventional power supply system. A DC/DC converter 127a, 127b is arranged between the secondary batteries 122a, 122b and the inverter 125, so that the discharging powers of the secondary batteries 122a, 122b are controlled to match the input voltage to the inverter 125. In other words, the output voltages of the solar battery 121a and the solar battery 121b need to be set to be equal, but the voltages of the secondary battery 122a and the secondary battery 122b are stepped up or stepped down to match a predetermined input voltage of the inverter 125 by the DC/DC converters 127a and 127b. All the storage batteries will have the same voltage according to such control, and thus one storage battery will not be overly discharged.
In charging to the secondary batteries 122a, 122b, a switch for bypassing the DC/DC converters 127a, 127b is arranged in advance, and a bypass switch is turned ON for the storage battery to be charged when solar irradiation is sufficient to charge from the solar batteries.
Therefore, the power supply system generally uses a DC/AC power conversion device to connect a power generator which uses natural energy such as solar battery and wind power generator, or a fuel battery to an electrical load or a power system. However, the DC/AC power conversion device is generally known to lower the conversion efficiency in time of low output.
Thus, as shown in FIG. 33, the conventional power supply system has outputs of a plurality of solar batteries 131a, 131b, and 131c connected so as to be collected at one junction box 132. Thereafter, a load 134 or a commercial system power 135 is connected by way of a plurality of parallel connected inverters 133a, 133b, and 133c, the operation of the inverters 133a, 133b, and 133c is controlled by a control unit 136.
The system of controlling the inverters is used in a household solar battery system or a large-scale solar battery system having a generated power amount of a few 10 kW to a few MW class.
The system shown in FIG. 33 is disclosed as a technique of connecting a plurality of small output inverters in parallel, and reducing the number of operating inverters when the generated power amount of the solar battery is small in Japanese Laid-Open Patent Publication No. 6-165513.
Japanese Laid-Open Patent Publication No. 2000-305633 discloses a technique of selecting the inverter to operate according to a predetermined rule such as in the order of small output power amount of each inverter or in the order of small operating time, or at random.
Japanese Laid-Open Patent Publication No. 2004-146791 discloses a power supply system, including a plurality of DC power supply strings for storing a generated power output of a plurality of DC power supplies including a solar battery, a fuel battery, or the like in each storage battery, for outputting power as necessary.